Related: The Browser Main Thread and Rendering Pipeline explains the full rendering pipeline that critical CSS inlining is optimizing for.

Lighthouse flags "eliminate render-blocking resources" and most developers look at their CSS files with mild confusion. The file is small. It is on a CDN. How can 15KB of CSS be blocking the render of a page that otherwise looks fast? The answer is in the word "render-blocking" itself, which is more precise than it sounds. The browser will not draw a single pixel to the screen until it has read every CSS file in the document head.…